1. Technical Field
This description generally relates to the field of automotive tire pressure sensors, and more particularly to sensors that provide the tire pressure while the automobile is being driven.
2. Description of the Related Art
The accurate measure of vehicle tire pressure while a vehicle is moving can prevent accidents and increase gas mileage. Government and university studies have cited the connection between tire under-inflation and vehicle crashes, including fatality rates.
On-board, in-motion systems for monitoring tire pressure exist, but typically these systems suffer from problems such as limited battery life, large size, and high cost. In systems that use a wired connection to power a tire-mounted sensor, there is the difficult problem of making a reliable continuous-path connection between the tire-mounted sensor and the chassis-mounted portion of the system. Monitoring the pressure sensor in such a system may also be a problem for the same reason.
One reference, “Solving the Last Milli-Mile Problem in Vehicle Safety; The EoPlex Approach to Powering Wireless Tire Pressure Sensors,” by Arthur L. Chait, discloses a potential alternative. The paper discusses a tire-mounted pressure sensor that transmits data from a rotating tire using a wireless signal. In one embodiment, the sensor and the transmitter are powered by a battery. In two alternative embodiments from this paper, the sensor is powered from within the tire by one of two kinds of energy harvester, the first one a generator within the tire wall that extracts energy from the rotation of the tire itself and the other a piezoelectric generator that extracts energy from tire vibration.
Generally speaking, tire pressure sensors can make a “direct” or “indirect” measurement. Pressure monitoring systems in the direct-type category employ physical pressure sensors inside each tire and a means of processing and sending that information from inside the tire to a remotely located vehicle instrument cluster. The sensors are mounted on either the end of valve stems or by a steel band around a rim's drop well center. The sensors are typically quite large and are subject to damage during tire removal and fitting procedures. Banded sensors may also damage the tire bead's air seal.
Tire pressure monitoring systems in the indirect category measure the “apparent” air pressure by monitoring individual wheel rotational speeds or other signals available from outside the tire itself. Indirect monitoring systems use the fact that an underinflated tire has a slightly smaller diameter than a correctly inflated tire and therefore rotates at a higher angular velocity to cover the same distance as a correctly inflated tire. Indirect pressure monitoring systems are generally cheaper and easier than direct measuring systems because most modern vehicles already have wheel speed sensors for antilock breaking systems and electronic stability control systems. A disadvantage of these systems is that they are not as accurate and require frequent recalibration. Of course, as the tire wears out, or if new tires of a different size are purchased, the system either needs to be recalibrated or is no longer sufficiently accurate to be useable.
An online EE Times article from Jun. 26, 2008, by David Carey discloses a direct measurement pressure monitoring system that provides a pressure reading over a wireless link powered by a battery. The wireless link operates in the 315 MHz band and power is provided by an internal coin cell battery with up to 10 years of life. The pressure reading is made by a single packaged two-chip component from SensoNor. The etched silicon MEMS transducer uses a silicon “drum head” strain gauge to sense pressure through an open port on the package top side. An Infineon brand transmitter chip modulates outputs from the sensor with either amplitude shift or frequency shift keying. A wire wound antenna hangs directly from the transmitter chip and a 9.8 MHz crystal supplies timing for the sensor/transmitter pair.
The February 2008 issue of IEEE Spectrum discloses a tire pressure monitor for the prevention of car rollovers. The pressure sensor is powered by an inexpensive coin-sized device called a PZT bimorph that harvests energy from the tire's motion via a miniature piezoelectric springboard. The power source can be used in a pressure sensor located internal to an automobile tire.